The Atacama Desert, a vast plateau stretching across northern Chile and parts of Peru, is the driest non-polar place on Earth. The aridity is so profound that some areas have no recorded rainfall, while the average annual precipitation in its hyperarid core can be less than 1 millimeter. The landscape is a result of a unique interplay of geographic and atmospheric factors. A massive mountain range, a powerful cold ocean current, and a persistent global weather system block almost all moisture from reaching the land.
The Rain Shadow Barrier of the Andes Mountains
The Andes Mountains, which run parallel to the Pacific coast, form the first major barrier to moisture reaching the Atacama. These mountains create a dramatic rain shadow effect, blocking moisture moving westward from the continent’s interior. The prevailing winds push humid air masses originating from the Amazon Basin and the Atlantic Ocean across South America.
As this moist air encounters the eastern face of the Andes, it is forced to rise, a process known as orographic lifting. The rapid ascent causes the air to cool and the water vapor to condense, resulting in heavy rainfall on the eastern slopes. By the time the air crests the mountains and descends onto the western side, it has been stripped of virtually all its moisture. This dry, sinking air warms adiabatically (due to compression), further lowering its relative humidity and promoting evaporation.
Atmospheric Stabilization by the Cold Humboldt Current
The second contributor to the desert’s dryness is the cold Humboldt Current, also known as the Peru Current, in the Pacific Ocean. This northward-flowing current originates in the Antarctic and moves frigid water along the coast of South America. The presence of this cold water chills the air mass directly above the ocean surface.
This chilling effect creates a persistent, low-level temperature inversion, trapping cold air beneath warmer air. This stable atmospheric structure prevents the air from rising and developing the deep convective clouds necessary for rain production. The only moisture that reaches the coast is a dense marine fog, locally known as Camanchaca. Although this fog provides a temporary water source for coastal ecosystems, the droplets are too small to precipitate as rainfall.
The Dominant Subtropical High-Pressure System
The final force governing the Atacama’s climate is the South Pacific Anticyclone, a persistent high-pressure system centered in the southeastern Pacific Ocean. High-pressure systems are characterized by air sinking from the upper atmosphere toward the surface, a process called subsidence. This sinking air compresses and warms, which significantly decreases its relative humidity.
The constant downward motion of this warm, dry air actively suppresses the formation of clouds and weather systems that could bring precipitation. The anticyclone also steers moisture-bearing storm tracks away from the coastal areas, further isolating the desert from potential rainfall. The combined effect of this subsiding air and the cold Humboldt Current ensures the Atacama Desert remains one of the most parched environments on the planet.